This invention relates generally to the rendering of latent electrostatic images visible. More particularly, the invention relates to non-interactive or scavengeless development systems and a method and apparatus for commutating power to electrodes of a toner donor roll which minimizes commutator induced wear of the electrodes.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoreceptor. The photoreceptor comprises a charge retentive surface. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves latent charge patterns on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction.
The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
The invention is particularly useful in highlight color imaging such as tri-level imaging. The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught in the '929 patent, the xerographic contrast on the charge retentive surface or photoreceptor is divided into three levels, rather than two levels as is the case in conventional xerography. The photoreceptor is charged, typically to -900 volts. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.cad or V.sub.ddp ). The other image is exposed to discharge the photoreceptor to its residual potential, i.e. V.sub.dad or V.sub.c (typically -100 volts) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically -500 volts) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100 volts closer to V.sub.cad than V.sub.white (about -600 volts), and the DAD developer system is biased about 100 volts closer to V.sub.dad than V.sub.white (about -400 volts).
The viability of printing system concepts such as tri-level, highlight color xerography requires development systems that do not scavenge or interact with a previously toned image. Since commercial development systems such as conventional magnetic brush development and jumping single component development interact with the image receiver, a previously toned image will be scavenged by subsequent development. Since the present commercial development systems are highly interactive with the image bearing member, there is a need for scavengeless or non-interactive development systems.
The present invention is especially suited for use in scavengeless single component development (SCD) systems wherein a confined toner cloud is formed in a 250 micron development zone gap by applying an AC bias of several hundred volts to one or more small diameter wire electrodes carried by a toner donor roll positioned adjacent a photoreceptor. The AC bias, which has a frequency is in the kilohertz range, acts upon the charged toner to induce a mechanical agitation which is sufficient to overcome adhesive forces that hold toner to the donor roll. Once freed, the toner is readily available to develop the electrostatic latent image on the photoreceptor.
In earlier renderings of this type of development system, the electrodes consisted of taut wires supported intermediate a photoreceptor and a toner donor roll. See for example U.S. Pat. No. 5,010,367 granted to Dan A. Hays on Apr. 23, 1991. Unfortunately, it has proven difficult to devise a mechanical design for the fragile taut wire array that is both robust, and free of development artifacts. For example, the wires tend to entrap toner agglomerates and spurious paper fibers which can cause streaks in the developed image.
The problems attendant taut wires may be obviated by using an array of small diameter wires or electrodes embedded in the surface of the donor roll. In this approach, the AC bias is applied to the wires in the development zone through commutating brushes at the ends of the donor roll. Such a construction is described in U.S. Pat. No. 3,996,892 granted to Parker et al on Dec. 14, 1976. The '892 granted to Parker et al on Dec. 14, 1976 discloses a spatially programmable electroded donor roll wherein an DC voltage is applied to the wire electrodes in the development nip or zone, pre-nip and post-nip zones through commutating brushes at the ends of the donor roll. Such an arrangement allows the bias profile around the circumference of a two component magnetic brush development roll to be tailored in a way that promotes good development. Thus, a pre-nip voltage of 100 volts, a nip voltage of 250 to 300 volts and a post-nip voltage of 1000 volts are provided. The electrodes on the donor roll were constructed by first plating a thin layer of copper on the outer surface a phenolic roll, and then by etching 0.01" wide electrode strips, on 0.02 centers, axially along the length of the roll. Next, the roll was overcoated with a semi-conductive rubber sheath, except for a short length at the ends where the bias was applied to the electrodes through commutating bushes. The voltage profile around the circumference of the roll was determined by the IR voltage drop due to current flow through the semi-conductive sheath from one or commutator to another. Such a construction is know to have had problems with wear and pitting of the thin electrodes where they made contact with the commutating brushes. Nickel plating the electrodes helped alleviate the wear problem somewhat, but the electrode damage problem was never completely solved.
The '892 patent, in a second embodiment, discloses the use of a ring-like resistive member mounted for rotation with a donor roll. A plurality of stationarily mounted electrical contacts ride on the ring-like member which, in turn, is seated on the coating free portions of conductors and mounted for rotation with a sleeve upon which the conductors are carried.
U.S. Pat. No. 4,568,955 granted to Hosoya also discloses a development or donor roll having electrode structures incorporated therein. Copper electrode structures are deposited on the insulated surface of a donor roll. In one rotational position of the Hosoya et al donor roll, a DC voltage is supplied to alternate ones of the copper electrodes while an AC voltage is supplied to the electrodes intermediate the electrodes having the DC voltage applied thereto. In another rotational position of this donor roll the AC and DC voltages are applied to the opposite electrodes. In other words, each electrode when positioned in the development nip first has one kind of voltage applied and then the other. The AC voltage establishes an alternating electric field for liberating toner particles on the surface of the donor roll. According to the Hosoya et al description, when the AC voltage is greater than the DC voltage the toner particles move from one electrode to an adjacent electrode and when the AC voltage is less than the DC voltage the toner particles move in the opposite direction between two adjacent electrodes.
U.S. Pat. No. 5,031,570 granted to Hays et al on Jul. 16, 1991 and assigned to the same assignee as the instant application discloses a scavengeless development system for use in highlight color imaging. AC biased electrodes positioned in close proximity to a magnetic brush structure carrying a two-component developer cause a controlled cloud of toner to be generated which non-interactively develops an electrostatic image. The two-component developer includes mixture of carrier beads and toner particles. By making the two-component developer magnetically tractable, the developer is transported to the development zone as in conventional magnetic brush development where the development roll or shell of the magnetic brush structure rotates about stationary magnets positioned inside the shell.
U.S. Pat. No. 4,868,600 granted to Hays et al on Sep. 19, 1989 discloses a scavengeless development system in which toner detachment from a donor and the concomitant generation of a controlled powder cloud is obtained by AC electric fields supplied by self-spaced electrode structures positioned within a development nip. The electrode structure is placed in close proximity to the toned donor within the gap or nip between the toned donor and image receiver, self-spacing being effected via the toner on the donor. Such spacing enables the creation of relatively large electrostatic fields without risk of air breakdown.
U.S. Pat. No. 5,010,367 granted to Dan A. Hays on Apr. 23, 1991 discloses a scavengeless/non-interactive development system for use in highlight color imaging. To control the developability of lines and the degree of interaction between the toner and receiver, the combination of an AC voltage on a developer donor roll with an AC voltage between toner cloud forming wires and donor roll enables efficient detachment of toner from the donor to form a toner cloud and position one end of the cloud in close proximity to the image receiver for optimum development of lines and solid areas without scavenging a previously toned image.
U.S. patent application Ser. No. 07/724,242 filed on Jul. 1, 1991 in the name of Dan A. Hays and assigned to the same assignee as the instant application discloses a scavengeless or non-interactive development system for use in image formation such as highlight color imaging. A toned donor roll structure having two sets of interdigitated electrodes physically supported by an insulative support structure is provided. Both sets of electrodes have a DC bias applied thereto while the other set has an AC bias applied thereto. The AC and DC biases are such as to preclude background development without creating fringe DC fields between adjacent electrodes.
U.S. Pat. No. 5,172,170 granted to Hays et al on Dec. 12, 1992 relates to an apparatus in which a donor roll advances toner to an electrostatic latent image recorded on a photoconductive member. A plurality of electrical conductors are located in grooves in the donor roll. The electrical conductors are spaced from one another and adapted to be electrically biased in the development zone to detach toner from the donor roll so as to form a toner cloud in the development zone. In the development zone, toner is attracted from the toner cloud to the latent image. In this way, the latent image is developed with toner.